Electrochemical cells are presently the preferred method of providing cost effective portable power for a wide variety of consumer devices. The consumer device market dictates that only a handful of standardized cell sizes (e.g., AA or AAA) and specific nominal voltages (typically 1.5 V) be provided. Moreover, more and more consumer electronic devices, such as digital still cameras, are being designed with relatively high power operating requirements. As has been the practice within the market, consumers often prefer and opt to use primary batteries for their convenience, reliability, sustained shelf life and more economical per unit price as compared to currently available rechargeable (i.e., secondary) batteries.
Within this context, it is readily apparent that design choices for primary (i.e., non-rechargeable) battery manufacturers are extremely limited. For example, the necessity of using specified nominal voltages significantly limits the selection of potential electrochemical materials, and the use of standardized cell sizes restricts the overall available internal volume available for active materials, safety devices and other elements typically expected in such consumer products. What's more, the variety of consumer devices and the range of operating voltages for those devices make smaller nominal voltage cells (which can be provided separately or in series, thereby giving device makers more design options) more versatile as compared to higher voltage electrochemical pairings typically associated with secondary batteries. Thus, 1.5 V systems, such as alkaline or lithium-iron disulfide systems, are far more prominent than others, such as 3.0 V and higher lithium-manganese dioxide.
Even among the 1.5V electrochemical systems (e.g., alkaline v. lithium-iron disulfide, etc.), the design considerations are significantly different. For example, alkaline and nickel oxy-hydroxide systems rely on an aqueous and highly caustic electrolyte that has a propensity for gas expansion and/or leakage, leading to very different approaches in terms of selection of internal materials and/or compatibility with containers and closures. In rechargeable 1.5 V systems (note that lithium-iron disulfide systems are not currently considered suitable for consumer-based rechargeable systems), various highly specialized electrochemical and/or electrolyte compositions may be used to best accommodate lithium ion charge/discharge cycling. Here, such high cost components are not a key design concern because secondary systems typically sell for a higher retail price than their primary battery equivalents. Moreover, the discharge mechanisms, cell designs and safety considerations are, by and large, inconsequential and/or inapplicable to primary systems.
Although lithium-iron disulfide cells display distinct advantages for high power devices (as compared to primary alkaline cells), LiFeS2 cell designs must strike a balance between the cost of materials used, the incorporation of necessary safety devices and the overall reliability, delivered capacity and intended use of the designed cell. In addition to improved capacity, cell designers must also consider other important characteristics, such as safety and reliability. Safety devices normally include venting mechanisms and thermally activated “shutdown” elements, such as positive thermal circuits (PTCs), while improvements to reliability primarily focus on preventing internal short circuits. In every example above, these desired traits require elements that occupy internal volume and/or design considerations that are usually counterproductive to cell internal resistance, efficiency or discharge capacity. Additional challenges are posed by regulations limiting the percent amount of weight lithium batteries can lose during thermal cycling, meaning that cell designs for smaller container sizes like AA and AAA can only lose milligrams of total cell weight (usually by way of evaporation of the electrolyte) even though the reactive and volatile nature of the non-aqueous electrolyte severely limits the universe of potential materials available.
The jellyroll electrode assembly is the preferred configuration in LiFeS2 systems. In order to effectively utilize iron disulfide in this configuration, the iron disulfide is mixed into slurry with the minimal amount of conductors and binders permitted to still effectively discharge the cell. This slurry is then coated and dried on a metallic foil current collector for utilization in the jellyroll, while the lithium is most effectively provided without a current collector. The separator is a thin polymeric membrane whose thickness is preferably minimized to reduce the inactive inputs into the cell. In order to maximize active materials, the anode will often consist essentially of lithium or a lithium alloy which doubles as a current collector along the entire circumferential length of the jellyroll.
Because the reaction end products of the lithium-iron disulfide electrochemical reaction occupy substantially more volume than the inputs, the electrode assembly swells as the battery discharges. In turn, swelling creates radial forces that can cause unwanted bulging of the cell container, as well as short circuits if the separator is compromised and/or disconnection within the anode itself. Previous means of handling these problems include using strong (often thicker) materials for the cell housing and inactive components within the cell. However, thicker inactive materials limit the internal volume available and thicker, more rugged electrodes were previously deemed not necessarily desirable because they allow for fewer winds possible in the jellyroll, resulting in less surface area between the electrodes and the expectation of comparatively lower performance at higher drain rates.
A number of approaches have been taken to strike an appropriate balance between optimal internal volume utilization and acceptable LiFeS2 cell capacity/performance. For example, a possible solution for problems created by swelling, disclosed in U.S. Pat. No. 4,379,815, is to balance cathode expansion and anode contraction by mixing one or more other active materials (such as CuO, Bi2O3, Pb2Bi2O5, P3O4, CoS2) with pyrite, although these additional materials can negatively affect the discharge characteristics of the cell, and the capacity and efficiency of the overall cell may also suffer. Other means of improving discharge capacity in LiFeS2 cell contemplate the use of thinner separators and/or specific cathode coating mixes and coating techniques, as disclosed in U.S. Patent Publication Nos. 20050112462 and 20050233214, or through the adjustment of interfacial input materials, as disclosed in U.S. Pat. Nos. 6,849,360 and 7,157,185.
Ultimately, improvements to capacity represent a fundamentally sound battery design. That is, in order to deliver greater capacity, careful consideration must be given for the radial expansion forces and other dynamics at work in a discharging lithium-iron disulfide battery. For example, if the design provides inadequate thickness in the anode or the cathode current collector then the radial forces during discharge may compress the jellyroll to such a degree so as to cause a disconnect in one or both electrodes and, once this disconnect occurs, the battery may cease to deliver capacity regardless of whether the active materials have all been discharged. Similar situations arise with respect to the void volume (in the cathode coating and the interior of the cell as a whole), the electrical connections throughout the battery, the separator, the closure/venting mechanism for the battery and the like. Therefore, the capacity of a LiFeS2 cell is a significant metric for the overall viability and robustness of a cell design, particularly when the cell designer is limited to the use of a standard-sized consumer battery (e.g., AA or FR6; AAA or FR03; etc.)
As a corollary to the capacity acting as a de facto metric for battery design, those skilled in the art will appreciate that design choices, and particularly the selection of specific components, must be made in consideration of the overall battery. A specific composition may have surprising, unexpected or unintended effects upon the other components and compositions within the cell. Similarly, in standard sized batteries, the selection of a particular element occupies volume within the container that might otherwise have been available for other elements. Thus, this interdependency of design choices necessarily means that any increase in capacity, and especially an increase that does not negatively impact the safety or performance of the battery in other regards, is much more than a simple act of adding more active materials. Similarly, the selection of inactive components, and particularly solvents, solutes, binders, conductors, polymers for the separator or seal and the like, must necessarily be made in context of the exigencies of the situation and in light of the vast multitude of choices, and isolating single items from an extensive list of possibilities is not by itself sufficient reason for an artisan to consider using in that particular combination.
Tests that simulate the actual discharge or use of the battery have particular relevance to evaluating cell designs. Typically, these simulated-use tests involve discharging the battery under specified discharge conditions (e.g., a constant load of 200 mA) continuously or in a predetermined cycle (e.g., discharge for a set number of minutes, followed by a rest interval of a set number of minutes) until the battery output voltage drops below a final “cut off voltage”. As used herein, tests that involve a cycle of discharge and rest intervals will be generically referred to as intermittent drain rate tests. Clearly, with any simulated use test, it is necessary to specify the discharge conditions, periods of time for discharge and the rest interval (if used) and the cut off voltage.
One particularly significant intermittent drain rate test is the ANSI Digital Still Camera Test (“DSC test”). The DSC test simulates the drain experienced by batteries inserted into a digital camera, which typically require short periods of high power demand while the user takes photographic images, followed by longer periods of inactivity. Accordingly, the DSC test for a AA sized battery involves discharging the battery at 1500 mW for 2 seconds followed by 650 mW for 28 second, and this 30 second cycle is repeated for 5 minutes every hour (i.e., 10 cycles/hour) followed by a rest period (i.e., 0 mW) for 55 minutes. Each 30 second cycle is intended to represent one digital still camera image. This one hour cycle is repeated every hour until the battery first records an output voltage of less than 1.05, although cell designers may occasionally continue the test beyond this end point to further observe battery discharge characteristics. The final performance is quantified in terms of number of minutes or number of images taken (i.e., the number of images will always be double the number of minutes on this test). Significantly, the cyclic, high-power requirements of this test make it one of the most difficult benchmarks for a battery design, while at the same time producing the most meaningful basis of comparison for battery consumers. Numerous other intermittent tests are known and utilized in this field, including those specified by the American National Standards Institute (“ANSI”), the International Electrotechnical Commission (“IEC”) and the like.